Great Vowel Shift

In Northern and Southern English, the first step of the Great Vowel Shift raised the close-mid vowels to become close. Northern Middle English had two close-mid vowels – /eː/ in feet and /øː/ in boot – which were raised to /iː/ and /yː/. Later on, Northern English /yː/ changed to /iː/ in many dialects (though not in all, see Phonological history of Scots § Vowel 7), so that boot has the same vowel as feet. Southern Middle English had two close-mid vowels – /eː/ in feet and /oː/ in boot – which were raised to /iː/ and /uː/. In Southern English, the close vowels /iː/ in bite and /uː/ in house shifted to become diphthongs, but in Northern English, /iː/ in bite shifted but /uː/ in house did not. If the vowel systems at the time of the Great Vowel Shift caused the difference between the Northern and Southern vowel shifts, /uː/ did not shift because there was no back mid vowel /oː/ in Northern English. In Southern English, shifting of /oː/ to /uː/ could have caused diphthongisation of original /uː/, but because Northern English had no back close-mid vowel /oː/ to shift, the back close vowel /uː/ did not diphthongise. See alsoCanaanite ShiftChain shift"The C